1. Field of the Invention
The present invention relates to an ion current detection device which detects the ion current caused by ionized combustion gas within an internal combustion engine in order to detect combustion states in cylinders of the engine.
2. Description of the Prior Art
An ignition type internal combustion engine (referred to as an engine hereinafter) produces power by compressing a fuel gas mixed with air in a combustion chamber (referred to as a cylinder hereinafter) with a piston and then igniting the mixed gas with an ignition plug. The power derived from the engine depends on the ignition timing with regard to the position of the piston. Generally an earlier ignition produces higher power. Too early an ignition, however, causes an abnormal combustion, referred to as knocking, which may result in damage to the engine. In order to avoid knocking, a conventional engine is provided with an oscillation sensor for detecting oscillation of the engine and determines an occurrence of knocking based on the signal from the oscillation sensor. The engine then delays the ignition timing so that an ignition occurs just outside the timing range where knocking would occur.
Knocking occurs in a cylinder. Detecting oscillation from a plurality of cylinders with high sensitivity largely depends on where the oscillation sensor is placed. The optimum position of the oscillation sensor for detecting the oscillation of all cylinders without interference from other oscillation, such as from the intake and exhaust valves, varies from one type of engine to another and is an important issue in engine design.
When combustion of the mixed gas occurs in the cylinder, the gas is ionized. Therefore, a voltage applied between electrodes placed in the cylinder causes an electric current to pass through it, which is referred to as the ion current. Since the ion current sensitively varies depending on the states of combustion in the cylinder, the measurement of the ion current provides information about the states of combustion.
The ion current rapidly increases in magnitude after ignition, reaching an maximum in a short period of time, and then decays relatively slowly. When knocking occurs, an additional component of a few kHz is superimposed on the ion current. Therefore, detecting knocking includes extracting only the oscillating component caused by the knocking from the rapidly changing ion current.
As mentioned above, the ion current is produced by the ionized gas in combustion triggered by ignition. Thus, its magnitude varies depending on the ignition condition and the engine speed. Normally, the ion current lasts for a period between a few msec and a few tens of msec. The oscillating component superimposed on the ion current due to knocking starts a few hundred .mu.sec after the ion current starts and is large in the first half of the ion current duration period. Therefore, an efficient measurement of the superimposed oscillating component must be made as soon as the ion current starts to flow. Since the ion current increases very rapidly, the ion current entering a band-pass filter with a sharp filter characteristic formed with an operational amplifier, for example, will cause a ringing at frequencies near the peak frequency of the band-pass filter immediately after the ion current starts to rise if a high frequency cut filter is not used. It should be noted that if the band-pass filter characteristic is sharper, the ringing is generated more easily and lasts longer.
Thus the ringing creates signals similar to the oscillating component of the ion current at the output of the band-pass filter, and hence precludes an accurate measurement of the knocking signal. This means that the detection of the knocking cannot be made until the oscillating signal due to the ringing becomes sufficiently small compared with the oscillating signal due to the knocking. The knocking cannot be detected during this period of time. It is well known, however, that the band-pass filter, which is designed to minimize the duration of the ringing, cannot detect knocking.
The applicant of the present invention disclosed in the patent application HEI 7-163869 a system for detecting knocking which utilizes ignition plugs as electrodes to measure the ion current. This knocking detector exhibits less performance variation among different types of engines than an oscillation sensor and thus provides for an accurate control system which no longer requires the oscillation sensor.
FIG. 18 illustrates a circuit diagram of the aforementioned ion current detection device. Referring to FIG. 18, an ignition device 201 of the simultaneous firing type is connected to an ion current detection device 202, which comprises a detection voltage generation circuit 203, an ion current/voltage conversion circuit 204, an ion current threshold detection circuit 205, a timer 206, a masking circuit 207, and an ac component detection circuit 208.
The detection voltage generation circuit 203 charges a capacitor C1 by the counter electromotive force induced when the transistor T1 of the ignition device 201 shuts off the current flowing through the primary coil L1 of the ignition coil L. Then the charge stored in the capacitor C1 produces a positive voltage on the ignition plugs PG1 and PG2 after the ignition discharge has occurred. The ion current/voltage conversion circuit 204 converts the ion current signal, which the detection voltage generation circuit 203 creates by applying the positive voltages at the ignition plugs PG1 and PG2, to a voltage signal comprising a low frequency component with an almost constant voltage and a high frequency component caused by knocking.
The ion current threshold detection circuit 205 compares the ion current which the detection voltage generation circuit 203 creates by applying the positive voltages at the ignition plugs PG1 and PG2 with a predetermined threshold current to determine whether or not the ion current is larger than the threshold current. If it finds that the ion current is larger than the threshold current, it produces an ion current detection signal. The timer 206 transmits the ion current detection signal produced by the ion current threshold detection circuit 205 with a predetermined time delay to control the transistor T2.
The masking circuit 207 removes the high frequency component from the voltage signal obtained by the ion current/voltage conversion circuit 204 which cannot be removed by the operational amplifiers because of its poor circuit responsiveness. The masking circuit 207 then amplifies the filtered signal and sends the sufficiently large signal to the ac component detection circuit 208. The masking circuit 207 also mask-controls the voltage signal provided by the ion current/voltage conversion circuit 204 responding to the operation of the transistor T2. That is, the masking circuit 207 amplifies the signal from which the high frequency component is removed in a predetermined time delay after the ion current threshold detection circuit 205 detects the ion current larger than the threshold current. Then the masking circuit 207 sends the amplified signal to the ac component detection circuit 208, which, comprising a band-pass filter, extracts the knocking signal included in the high frequency component which was amplified by the masking circuit 207.
The circuit shown in FIG. 18 comprises a feedback circuit utilizing operational amplifiers which, when an ion current larger than the predetermined threshold current flows, produces a constant voltage regardless of the magnitude of the ion current and which has a small feedback gain for the signal with the specific frequency associated with the knocking. The feedback circuit enhances the relative magnitude of the knocking current component with the specific frequency within the ion current signal. The ion current signal, including the enhanced knocking current component, passes through the filter comprising capacitors and resistors. Then only the knocking signal is extracted from the ion current by means of the band-pass filter comprising operational amplifiers.
As indicated above, a highly efficient band-pass filter generates a ringing near the peak frequency when a rapidly changing signal such as a step signal is introduced to the filter. Therefore, processing of the ion current with the band-pass filter without preprocessing it gives rise to a ringing immediately after the ion current is generated, thereby resulting in a false detection of the knocking current causing the system to falsely determine that a knocking has occurred even if it has not.
Further, the ion current detection device shown in FIG. 18 requires many operational amplifiers. Therefore, it is large and expensive.
The ion current detection device shown in FIG. 18, having the capability to detect a knocking signal, shuts off the signal entering the band-pass filter immediately after the ion current is generated but allows for the signal to enter the band-pass filter after a predetermined time delay. The masking circuit 207, which shuts off the signal entering the band-pass filter, includes capacitors and resistors to smooth the signal entering the band-pass filter so that the signal does not change rapidly after the shut-off circuit ceases to operate. This configuration is complex, and a new configuration which is simpler and yet can avoid the ringing is desired.